This program project is comprised of four research projects each concerned with basic investigations of the human immune system related to immunological diseases. Project I will seek clarification of X-chromosome-linked genetic control of lymphoid development in man and, through this effort, a better understanding of X-linked primary immunodeficiency diseases may be achieved. Project II is concerned with the persistent but sterile form of arthritis that follows intestinal or urethral infection with selected bacterial pathogens exemplified by seronegative "reactive" arthritis and Reiter"s syndrome. Analysis of the T lymphocytes infiltrating the joints and bowel wall will be the major focus of this research. Project III will be devoted to an in-depth study of this significance of a newly recognized endogenous retrovirus-like human genomic locus which is apparently related to HTLV-I and may have potential to become activated in immunoproliferative diseases. Project IV will continue investigation on the expression and function of human platelet Fc(IgG) receptors and will initiate a study of Fc(IgG) receptors in human vascular endothelial cells. These investigations receive impetus from an interest in the potential role of immunologically activated platelets and endothelial cells in immune complex and vaso- obstructive diseases.